Its called corna, and its Italian for horns. Also known as mano cornuto, mano meaning hand. So, hand horn. In Italy there is a tradition of this, by putting it behind someones head in, lest say, a picture.

It is also found in many latin lands, in Argentina it has a tradition to get rid of bad luck. Like when a black cat crosses the street, get up that corna and shake of the bad luck. Argentina has a lot of Italian tradition, that explains the reason of them addapting it.

In satanism LaVey took this sign, but remember that satanism is new. It was started in the 60s, and we are just talking about a moron called LaVey trying to be cool.

In heavy metal Dio made it popular, and I see the page linked to even posting a albumcover as proof of some evil conspiracy. So, you belive Dio more then history?

It is also used in sports, like "hook em horns" and in baseball it means "two outs". In other cultures it also means other things, like mentioned in Italy, in Turkey its an old nation symbol.